Daily NO vertical column density (VCD) has been routinely measured byzenith sky spectroscopy at the subtropical station of Izaña (28° N,16° W) since 1993 in the framework of the Network for the Detection ofAtmospheric Composition Change (NDACC). Based on 14 years of data the firstlow latitude NO VCD climatology has been established and the maincharacteristics from short timescales of one day to interannual variabilityare presented. Instrumental descriptions and different sources of errors aredescribed in detail. The observed diurnal cycle follows that expected bygas-phase NO chemistry, as can be shown by the good agreement with avertically integrated chemical box model, and is modulated by solarradiation. The seasonal evolution departs from the phase of the hours ofdaylight, indicating the signature of upper stratospheric temperaturechanges. From the data record (1993–2006) no significant long-term trends inNO VCD can be inferred. Comparison of the ground-based data sets withnadir-viewing satellite spectrometers shows excellent agreement forSCIAMACHY with differences between both datasets of 1.1%. GOME displaysunrealistic features with the largest discrepancies during summer. Theground-based data are compared with long-term output of the SLIMCAT 3-Dchemical transport model (CTM). The basic model, forced by ECMWF (ERA-40)analyses, captures the observed NO annual cycle but significantlyunderestimates the spring/summer maximum (by 12% at sunset and up to25% at sunrise). In a model run which uses assimilation of satelliteCH profiles to constrain the model long-lived tracers the agreement issignificantly improved. This improvement in modelled column NO is dueto better modelled NO profiles and points to transport errors in the ECMWFERA-40 reanalyses.
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